We have two 80-gallon Stiebel Eltron heat pump water heaters in our ~2,000 sq. ft. garage with insulated garage doors. We're in south Florida where garage heat and humidity can get intense. How much cooling and humidity reduction you'll get from the water heaters depends somewhat on how much hot water you use. With only two people in the house most of the time, we don't use enough hot water to get a lot of heat and humidity relief in the garage, although some is detectable.
However, when one of the water heaters is running, the air blowing off the condenser coils into the garage is quite cool and dry. So a heat pump water heater in a small, well-insulated garage of a house that uses a lot of hot water would probably make a significant difference in the summer in that situation. (Remember, though, that such a water heater pulls heat from ambient air to heat the water. In a garage in a wintry climate, the heat pump will have to work harder and will actually cool the garage down further.)
My take: heat pump water heaters are the best environmental answer for non-solar water heating. They are only marginal contributors to climate control in most garage installations.